The present invention relates to inlet screens through which water flows to ocean thermal power plants.
In an ocean thermal power plant, a large quantity of warm water must be injected into the plant from the ocean surface for generating a given amount of power. Typically, for a power plant that generates 100 MW of electricity, the total volume flow of water from close to the surface in the ocean will be on the order of 250 cubic meters per second. This water must flow through a pumping system and a deaeration system before passing through shell and tube heat exchangers in which the tubes have an inside diameter of, for example, about 2 inches. The water flowing into the plant must be screened to keep out fish, jellyfish, and any other floating objects in the sea.
In order to prevent fish from being sucked against a screen, it is generally acceptable practice that, if velocities into the screen are kept as low as 0.5 feet per second or less, then fish will not be sucked against the screen to plug it up. The openings in a screen are sized such that any object large enough to plug the tubes in the heat exchangers should not be allowed to enter through the screen. A typical practice that is considered practical is to limit the size of the openings in the screen to an opening one half the diameter of the bore of each tube. For example, if the bore of each tube is 2.032 inches, then the opening in the screen should not be more than 1.016 inches.
By the present invention, an ocean power plant inlet screen is provided that has openings sized to keep the velocities of water flowing through the screen below a level that would cause fish to be sucked against the screen to plug it up. The openings in the screen are also sized to keep out all objects that are large enough to plug the tubes in heat exchangers within the ocean thermal power plant. Also in accordance with the present invention, a screen is provided which is as nearly self-cleaning as possible. Furthermore, the structure of the screen assures that all objects too large to pass through the screen are stopped at the outer surface of the screen so that they cannot become wedged in the screen farther into flow passages defined by the screen openings. Moreover, a screen is provided which damps out ocean waves as they strike the screen. As a result, the waves impose less force on the ocean thermal power plant than they would if they struck a blank wall of the power plant.
In order to provide the advantages described above, the inlet screen according to the present invention comprises a plurality of parallel bars extending at an angle to the horizontal. The screen is oriented parallel to the current in the ocean, with the bars angled back in the upstream direction. As a result, the ocean current tends to sweep off any objects that catch on the screen and to send them downward into the ocean. Furthermore, since bars are easily swept free of debris, mechanical cleaning with a rake is facilitated.
The total area of the openings in the screen is sized to keep the velocities of ocean water entering the screen at no greater than 0.5 feet per second, so that fish will not be sucked against the screen to plug it up. Furthermore, the spacing between adjacent parallel bars is made such that the distance between bars in the vertical direction is less than half the inner diameter of the tubes in the heat exchangers. Since the vertical height of most fish is greater than their horizontal width, fish higher than one half the inner diameter of the heat exchange tubes are stopped by the inlet screen.
The width of each bar of the inlet screen is greatest at the outer side of the screen, facing the sea, and the bars taper to a narrower width at the opposite, inner side of the screen. As a result, any objects small enough to enter an opening in the screen will not become caught or wedged between bars farther into the screen. Instead, objects small enough to enter the openings between bars at the outer side of the screen move freely through the rest of the screen.